So, I put the thing together, hooked a remote to the back, and fed it CO2. It promptly tried to vent the entire bottle through the barrel. Turned the CO2 off, cocked it, and turned it back on. No leak whatsoever.

Are these guns supposed to be cocked before you give them air? What difference would that make?

Spring tension from the main spring. It's pushing the hammer against the valve, opening it a little. You take cock the hammer back, the valve spring closes the valve and lets the valve chamber pressurize. Once that initial pressure is in the chamber, the hammer resting against the valve doesn't have enough force to push it open.